1. Field of the Invention
The present invention relates to a DC-DC converter used in communication apparatus and the like. More particularly, the invention relates to a DC-DC converter that has improved transient response characteristics.
2. Description of the Related Art
DC-DC converters which convert an input DC voltage into a desired DC voltage and output the latter are widely used in communication apparatus and the like. For example, such DC-DC converters have a series circuit of a main switch and a choke coil and output a desired DC voltage to a load via an output capacitor.
FIG. 7 is a block diagram showing the configuration of a general DC-DC converter of such a type. An input voltage Vin from a DC power source 101 is converted by a converter section 102 into a desired output voltage Vout, which is supplied to a load 103. The converter section 102 includes an output stage and an LC filter and is composed of a coil, switches, a rectification element, a capacitor, etc.
A control circuit 110 which controls the converter section 102 is composed of a detection circuit 111 for detecting the output voltage Vout of the converter section 102, an error amplifier 113 for comparing a resulting detection voltage Vo with a reference voltage Vref supplied from a reference voltage source 112, and a comparator 115 that compares an output Ve of the error amplifier 113 with an output Vosc of an oscillator 114, and supplies a control signal Vcont to the converter section 102.
FIG. 8 shows the configuration of the control circuit 110 of the above conventional DC-DC converter. The detection circuit 111 is composed of voltage division resistors R101 and R102, and an output of the comparator 115 is input to the converter section 102 via a logic circuit 116. A series circuit of a resistor R103 and a capacitor C101 is connected between the inverting input terminal and the output terminal of the error amplifier 113.
FIGS. 9A and 9B are circuit diagrams showing exemplary configurations of the converter section 102 of the above conventional DC-DC converter. Each of these circuits constitutes a step-down converter. The circuit of FIG. 9A is provided with a series circuit of a choke coil L101 and a main switch S101. The main switch S101 is turned on or off by a drive circuit 120. An output capacitor C111 is connected to one end of the series circuit (i.e., the terminal of the choke coil L101 that is not connected to the main switch S101). A switch S102, which is turned on or off by the drive circuit 120, is connected to the connecting point of the main switch S101 and the choke coil L101. In the circuit of FIG. 9B, a diode D101 is used in place of the switch S102 used in the circuit of FIG. 9A.
In the DC-DC converter having the above converter section 102 and control circuit 110, when the output voltage Vout is changed by changing the reference voltage Vref, the response speed may be lowered due to occurrence of an overshoot or the like. A known DC-DC converter whose control circuit has an error amplifier is improved to increase the response speed when the output voltage is changed by changing the reference voltage, in particular, to settle the output voltage at high speed by reducing an overshoot (see, for example, JP-A-2002-78326 or the corresponding U.S. Pat. No. 6,469,483). In this DC-DC converter, the error amplifier is formed by cascade-connecting a differential amplification circuit and an analog amplification circuit both having a fixed operating point.
FIGS. 10A-10C are graphs showing frequency characteristics of the converter section 102 and the control circuit 110 of the conventional DC-DC converter of FIG. 7. Relationships between the gain (dB) and the frequency, and relationships between the phase (degrees) and the frequency are shown. FIG. 10A shows characteristics of the converter section 102, FIG. 10B shows characteristics of the control circuit 110, and FIG. 10C shows characteristics of the combination of the converter section 102 and the control circuit 110.
The converter section 102 exhibits characteristics of a low-pass filter that is composed mainly of a coil and/or a capacitor. The frequency characteristics of the control circuit 110 are adjusted so that the gain of the entire loop including the converter section 102 is made stable. In general, the operating frequency band of the control circuit 110 is set lower than that of the converter section 102 and its bandwidth is several kilohertz or less. Therefore, the frequency bandwidth of the entire loop including the converter section 102 and the control circuit 110 is also several kilohertz or less. The time constant T of a transient operation of the DC-DC converter is given by T=½πf (longer than several microseconds), where f is the above-mentioned bandwidth. In the DC-DC converter disclosed in JP-A-2002-78326, although the settling time is shortened by suppressing an overshoot, and the response waveform and the response time are improved, the response time is restricted by the frequency bandwidths of the control circuit and the converter section.
One method for eliminating the restriction due to the frequency bandwidth of the control circuit is to change the time ratio of the main switch S101 (see FIGS. 9A and 9B) stepwise at the same time that the target value of the output voltage is changed. This method can change the output voltage forcibly, independently of the response of the control circuit. However, in this case, the response of the control circuit is determined by the frequency characteristics of the DC-DC converter, that is, a damping constant and a phase constant that are determined by the loss resistance of the main switch S101, the inductance and the loss resistance of the choke coil L101, the capacitance and the loss resistance of the output capacitor C111 (see FIGS. 9A and 9B), and the load (i.e., the load 103 shown in FIG. 7).
To make the frequency range of the frequency characteristics of the above-described conventional DC-DC converter higher, it is necessary to decrease the inductance of the choke coil and the capacitance of the output capacitor. However, as the inductance of the choke coil and the capacitance of the output capacitor decrease, the AC loss increases due to an increase of the ripple in the output voltage and an increase of the AC current flowing through the choke coil, and the efficiency of the converter section thereby is lowered. Furthermore, to suppress an overshoot in a stepwise operation of the DC-DC converter, it is necessary to increase the loss of the converter section to suppress transient oscillation. This results in a problem that the efficiency of the converter section is lowered.